Undersea optical fiber cables have evolved over many years to current designs in which large numbers of wavelength division multiplexed (WDM) channels provide very high transmission capacity. To reach this result the optical fiber properties are carefully managed to avoid excessive accumulated dispersion, adverse non-linear effects such as four-wave mixing, excessive transmission loss between amplifiers, excessive splice loss at each amplifier stage, etc. Typical state-of-the-art undersea cables employ transmission lengths of negative-dispersion non-zero dispersion fiber (NZDF) alternated with dispersion compensating fiber (DCF) to control the amount of accumulated dispersion over the entire transmission length. Earlier undersea cables were designed with “blocks” which covered several amplifier span lengths, i.e. the distance between amplifiers. A block would have several sequential lengths of negative dispersion transmission fiber, and would accumulate considerable negative dispersion, at which point the block would terminate with a length of DCF (positive dispersion) fiber. To keep the accumulated dispersion low, the dispersion values of these fibers were deliberately low.
Introduction of advanced optical fiber amplifier protocols allows greater accumulated negative dispersion. Accordingly, newer undersea cable designs are “blockless” and have single lengths of negative dispersion coefficient transmission fiber alternated with single lengths of DCF fiber. Accumulated dispersion values are allowed to be higher, since each alternate increment of the transmission fiber is compensated. The more advanced of these cable designs typically also add a new feature, management of dispersion slope. The dispersion slope of the negative-dispersion transmission fiber is slightly negative. This is matched with a compensating fiber with positive dispersion slope. The design of these cables is complex, since both properties—dispersion and dispersion slope—are managed simultaneously. Manufacture of these cables is also complex, with added cost. Adding further to the complexity is the relatively small effective area of negative dispersion, dispersion-slope managed optical fiber, and the relatively large attenuation. These properties have negative consequences for both transmission loss (dB per km) and also for splice loss. The overall design therefore results in significant added end-to-end loss in the cable. To preserve signal strength in the face of that added loss requires additional amplifiers, at considerable added expense. It was expected that managing the dispersion slope would provide an acceptable trade-off for added loss. However, in many cases that may not prove to be the case.
New undersea cable designs that deal more effectively with the trade-off between various optical fiber properties, and provide transmission capacity comparable to dispersion slope managed cables but at lower cost, would be desirable.